DevTools: [CodeMirror] undo/redo also centers line

This patch improves undo/redo behaviour to center the
changed line on screen.

BUG=
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/19471014

git-svn-id: svn://svn.chromium.org/blink/trunk@154571 bbb929c8-8fbe-4397-9dbb-9b2b20218538